#include<iostream>
#include<iomanip>
using namespace std;

int main()                               //牛吃草问题
{
    double x,a,y,b;
    cin >> x >> a >> y >> b;
    double z = (b*y - a*x)/(b - a);
    cout << fixed << setprecision(2) << z << endl;
    return 0;
}